This invention generally relates to pneumatic rotary tools and more particularly to an improved pneumatic rotary tool having a plastic housing and a variable torque design for efficient use of pressurized air.
The invention is especially concerned with a powered tool that rotates an output shaft with a socket for turning a fastener element such as a bolt or nut. Tools of this type are frequently used in automotive repair and industrial applications. Conventionally, pneumatic rotary tools comprise a metallic outer housing with multiple metallic internal parts. These tools are strong and durable due to their metallic construction, although the all-metal construction makes them both somewhat heavy and costly. Pressurized air flowing through the tool powers tools of this type. As the air expands within the tool, it induces motion of an internal motor, powering the tool.
It is an aim of tool manufacturers to provide a pneumatic rotary tool that is as durable as an all-metal tool, but employs portions formed from lighter materials, such as plastic, where appropriate to reduce the weight and cost of the tool. One difficulty in the design of such a tool is the reduced rigidity of plastic as compared with a strong metal, such as steel. For instance, should a plastic tool fall against a hard surface, a metallic air motor inside the tool may shift and become misaligned, or canted, with respect to the housing and the output shaft, rendering the tool unusable. This problem has led tool manufacturers to create complex internal motor casings designed to inhibit the motor from canting in the housing. For example, U.S. Pat. No. 5,346,024 (Geiger et al.) discloses such a motor casing, described as a motor cylinder 15. This casing is cylindrical in shape, with one closed end that includes multiple parts, such as a back head 26 and bore 27, extending from the closed end. The cylinder, back head and bore are of unitary construction, making a closed end cylinder significantly more difficult to manufacture. Therefore, these casings are expensive to manufacture, which may mitigate the cost benefit of using lighter and less costly materials, such as plastic, for other parts. As such, a tool formed inexpensively from both lightweight material and metallic parts is desirable.
In addition, conventional rotary tools often incorporate mechanisms to regulate torque according to user input. One such tool uses back pressure within the air motor to regulate the torque output. As backpressure within the motor increases, the torque output of the motor decreases. Such a design is inefficient because it uses the maximum flow of pressurized air to power the tool, while operating below its maximum power. At lower torque settings, a large portion of air bypasses the motor for backpressuring the motor, adding no power to the tool. As such, a tool that can more efficiently regulate torque by using less pressurized air is needed. Moreover, a tool that can reduce backpressure in the motor will operate more efficiently, using less air for the same work.
Typically air motors incorporate a rotor having a plurality of vanes upon which the pressurized air can react, inducing rotation of the rotor. Pockets of pressurized air are received within compartments defined by adjacent vanes. Conventional rotary tools typically have a single exhaust port in the air motor for exhausting pressurized air from the motor. As each rotor compartment passes the exhaust port, much of the air within the compartment passes through the exhaust port and exits the motor. Any air remaining within the compartment after the compartment passes the exhaust port becomes trapped within the compartment. The volume of the compartment decreases as the compartment nears completion of a motor cycle, and the compartment must compress the air within the compartment for the rotor to continue to rotate. Compressing the air within the compartment (backpressure) reduces the rotational speed of the turning rotor. Backpressure reduces motor efficiency; thus, a pneumatic rotary tool that reduces backpressure losses within the air motor is desirable.